Birth related injuries are rare but devastating events because the consequences can lead to lifelong impairment for the baby, family and society in general. During labor, clinical staff monitor various health characteristics of the obstetrics patients in order to obtain a qualitative assessment of the mother's and the fetus's well-being.
Fetal oxygen deprivation during labor and delivery can affect the fetal brain and result in permanent brain injury or even fetal death. A challenge for clinicians caring for women during childbirth is to identify the small number of babies who are experiencing clinically significant hypoxemia in order to intervene before there is progression to fetal brain injury, without causing an excessive number of unnecessary interventions in the great majority of women with normal childbirth. This problem is challenging because clinicians have limited access to the babies during labor and cannot measure fetal brain oxygenation directly.
Various solutions have been proposed to address this challenge. An approach commonly used by clinicians is to rely upon specialized devices that can measure the fetal heart rate and uterine pressure. The fetal heart rate and uterine pressure are then displayed as tracings over time and visually examined by the clinicians to make inferences about the state of the baby. A deficiency with such an approach is that the display of the fetal heart rate over time does necessarily fully reflect the state of the fetal brain and visual inspection has poor resolution of fine details and relationships between the signals.
Heart rate is constantly modulated by the sympathetic nervous system (increasing the heart rate) and parasympathetic nervous system (decreasing the heart rate). These brief changes in the baseline heart rate are known clinically as heart rate variability. Trends in heart rate variability and the degree and duration of minimal or absent variability are considered as material clinical indicators of fetal cerebral state. For instance, persistently low fetal heart rate variability is thought to represent an energy conserving state of the fetal brain in response to hypoxemia or acidemia or to reflect actual brain injury. Experimentation using pharmacologic blocking agents has shown that the parasympathetic nervous system mostly influences the high frequency (HF) or short term component of heart rate variability and the sympathetic nervous system mostly influences the low frequency (LF) or long term component of heart rate variability. Fetal movement is associated with changes in the medium frequency (MF) range. As such, measuring these frequency components of heart rate variability is advantageous clinically because it provides information on specific regions of the brain.
Currently used methods for measuring fetal heart rate variability have limitations. In particular, clinicians typically measure variability by visually inspecting the fetal heart rate tracings using the gridlines on the fetal heart rate monitor paper printout. Following the definition of variability provided by the National Institute of Child Health and Human Development (NICHD) in 1999, a common approach is to measure the amplitude of fluctuations around the baseline fetal heart rate. The baseline fetal heart rate are segments of the recording where the fetal heart rate is relatively constant or flat and is without accelerations (transient increases in the heart rate lasting 15 seconds to a few minutes) or decelerations (transient decreases in the heart rate lasting 15 seconds to a few minutes). The baseline variability is then classified as i) absent—amplitude range undetectable; ii) minimal—amplitude range detectable: 5 beats per minute or lower, iii) moderate: 6-25 beats per minute, or iv) marked: >25 beats per minute. Numerous studies have shown poor levels of agreement on visual estimates of absent or minimal variability. Another deficiency associated with visual inspection to measure heart rate variability is that the human eye can generally not accurately separate and measure the components of heart rate variability across its frequency spectrum. As such, techniques based on visual inspection of a heart rate signal to measure heart rate variability are deficient in that: i) they are limited to a simple measure of FHR excursions around the baseline segments; ii) show poor agreement levels in the presence of low variability; and iii) cannot distinguish between components of fetal heart rate variability in specific frequency bands.
Although some techniques for measuring heart rate variability from a digital heart rate signal have been proposed in the past, measuring fetal heart rate variability alone or its components does not fully resolve the clinicians' challenge to identify which babies are experiencing clinically significant hypoxemia in order to intervene before there is progression to fetal brain injury. In particular, it is noted that while low heart rate variability may be caused by a number of pathological conditions, low heart rate variability may also be caused by benign conditions. In fact, a number of benign conditions can affect heart rate variability such as fetal sleep-wake cycles or medications which are not harmful. Pathological conditions such as remote brain injury earlier during the prenatal period which are not improved by intervention during labor can also cause low heart rate variability. As such existing techniques using fetal heart rate variability may not be adequate for distinguishing between benign and hypoxemic conditions.
In light of the above, there is a need in the industry to provide methods and devices for monitoring a baby in-utero during labor that alleviate at least some of the above identified deficiencies.